Effects of 5-minute cerebral ischemia due to bilateral occlusion of the common carotid arteries were studies in 3-week old and adult gerbils. The biochemical assays revealed a considerable difference concerning the rate of depletion of the main energy metabolites indicating a slower energy metabolism in the young gerbils. Morphological studies carried out after 2 weeks revealed no evident ischemic injury in the young animals, whereas the brain of adult gerbils showed characteristic severe destruction of the CA1 sector of the hippocampus. The evaluation of regional cerebral blood flow (rCBF) with 14C iodoantipyrine radioautography revealed severe, uniform (below 10 ml/100g/min) ischemia in most of the both hemispheres, similar in intensity in both young and adult gerbils. The quantitative rCBF measurements based on hydrogen clearance and using implanted platinum electrodes indicated a marked hypoperfusion demonstrable at 30' and 1 hr recirculation periods in adult animals, whereas this was not present in young gerbils. Oxygen availability determinations indicated at 30 and 60 minutes recirculation a tissue hypoxia, much more pronounced in the adult than in the young animals. The studies indicate that among factors accounting for better tolerance of ischemia by young gerbils, in addition to slower energy utilization during initial stages of ischemia, considerably less pronounced tissue hypoxia during post-ischemic periods may play a significant role in pathomechanisms of ischemic injury.